The present invention relates to mobile radio communication systems such as an analog cordless telephone system, a digital cordless telephone system, a digital intercommunicating radio LAN system, an analog automobile telephone system and a digital automobile telephone system.
In recent years, there have been proposed a cordless telephone system which allows a user to use a cordless telephone while walking in a building, factory or indoors and outdoors, and an automobile telephone system which permits a telephone call and data transmission from an automobile being driven at a high speed; these systems are now being rapidly put to practical use. In existing systems, base stations individually have equipments such as a MODEM and a radio signal monitor set and a communication control station which presides over a plurality of base stations takes charge of channel assignment and channel handover. In a case where the service area is far and wide, however, the cost of system constructed on will rise high because of the necessity of installing a number of base stations and the load on the communication control station will also increase substantially because of simultaneous control of many base stations. Furthermore, in a TDMA/TDD (Time Division Duplex "Ping-Pong transmission") system such as a PHP (Personal Handy Phone) system, precise synchronization between base stations is needed, but the larger the number of base station, the harder their synchronization.
One possible solution to this problem is the configuration of a base station for mobile communication disclosed in a literature "Studies of Dynamic Channel Assignment in Communication between Base Stations which Uses Sub-Carrier Transmission" (Ogasawara, et al., 1992 Spring Meeting of Institute of Electronics, Information and Communication Engineers of Japan, B-315). The proposed configuration is to share a MODEM among a plurality of micro cells. In the above-mentioned literature a macrocell base station has such a configuration as shown in FIG. 8. Reference numeral 10 denotes a macrocell base station, 11 through 1n n MODEMs, 2 a switching circuit for interconnecting the MODEMs and microcell base station antennas, 31 through 3m m microcell base station antennas, 4 a connection control circuit for controlling the MODEMs 11 through 1n and the switching circuit 2 to conduct a signal of a specified channel between a specified MODEM and a specified antenna, 5 a channel assignment control circuit which allocates/deallocates the MODEMs 11 through 1n and channels in accordance with circuit allocation/deallocation and indicates to the connection control circuit 4 the correspondence between the MODEMS 11 through 1n, the channels and the microcell base station antennas 31 through 3m, 8 mobile station and 91 through 9m microcell zones which are covered by the microcell antennas 31 through 3m.
Moreover, there is proposed in a literature "Radio Signal Collection/Distribution System for Microcell Mobile Communication Using Optical Fiber Transmission" (Shibuya, et al., RCS90-12) an optical microcell system which interconnects each microcell base station and a macrocell base station with optical fiber cables, in contrast to the above-mentioned wire microcell base station concentration system employing the sub-carrier transmission. This system transmits an optical signal over the optical fiber cables after converting thereto a radio-frequency (RF) signal in the conventional radio section by means of electro-optic (E/O)/opto-electric (O/E) signal converters installed in each of the microcell and macrocell base stations. In this system, each microcell base station needs only to have an antenna for signal transmission and reception use, an O/E-E/O converter and a power amplifier, and a MODEM and other radio equipment can collectively be provided in the macrocell base station. This permits substantial miniaturization of the microcell base station and affords a substantial reduction of its cost. Besides, in the case of changing the zone configuration, reshuffle and relocation of microcell base stations can easily be done and software modification of the macrocell base station can also be effected under centralized control. Hence, the optical microsystem utilizing optical fiber cables can be said to be a promising next generation microcell system which is highly excellent from technical and economical points of view as well as in terms of system configuration.
According to the optical fiber microcell base station concentration system utilizing the sub-carrier transmission, in the case of establishing a channel assignment control circuit 5 allocates the MODEM and the channel to be used and indicates to the connection control circuit 5 a combination of the allocated MODEM and channel and an antenna which covers the microcell zone where the mobile station is located. Based on the combination of the MODEM, the channel and the antenna indicated from the channel assignment control circuit 5, the connection control circuit 4 controls the transmitting/receiving frequency of the MODEM and the connection between the MODEM and the antenna via the switching circuit, that is, the connection control circuit 4 operates so that a signal of the specified channel is transmitted between the specified MODEM and the specified antenna.
As regards a method for switching zones when the mobile station moves from one zone to another, there has not been proposed so far any particular method suitable for use with this system configuration. Supposing use is made of a method which, after detecting the necessity of zone handover in the macrocell base station or mobile station, performs zone handover following a predetermined procedure between the base station and the terminal equipment as is the case with a system having an individual MODEM for each microcell zone, the mobile station also needs to have a processing feature for zone handover.
As mentioned above, in the case of employing conventional base stations for mobile communication, although MODEMs are shared among zones, it is necessary that when moving from one zone to another, the mobile station perform zone handover through the same procedure as in the system having a MODEM for each zone; thus, the mobile station needs to follow the zone handover procedure and the zone handover takes so much time that there is a possibility of a break occurring in communication.